This invention relates to a method of selectively recovering valuable metals from metal-rich industrial solid wastes and scrap home electric appliances, in particular, those processed wastes which contain both iron and nonferrous metals (chiefly copper). More specifically, this invention relates to a method by which valuable iron, copper and other nonferrous metals can be selectively recovered in a simple and economical manner from motor scrap, in particular, small motor's scrap, or shredded automobiles and home electric appliances, which are disposed of in large tonnage every year. Such industrial solid wastes and scrap home electric appliances have a high content of both ferrous and nonferrous metals and should be used as precious industrial raw materials for countries in which natural resources are scarce, such as Japan. However, in the absence of established economical processing methods that are competitive on the market, those industrial solid wastes and scrap home electric appliances are not currently used as a source of ferrous or nonferrous raw materials but are simply disposed of as landfill materials.
With the recent advances in the industries of automobiles, home electric appliances and office-automation equipment, various small-sized motors have come to be used in a broad range of fields. Most of the wastes that originate from such small motors are disposed of together with abandoned large motors that are derived from the heavy-industry sector. In total, the quantities of motor scrap that is annually disposed of after use are immense.
Of the two types of motor scrap, the scrap of large motors is disassembled manually and, after the copper wires are removed fairly easily, the remainder is selectively recovered as waste copper and iron scrap. However, the product recovered in this way accounts for only a very small portion of the total quantity of motor scrap that is disposed of.
Various methods have been proposed for selective recovery of copper and iron from motor scrap and they include melting copper as a copper-aluminum alloy by treatment with molten aluminum, as well as passiveting iron with nitric acid so as to achieve selective leaching of copper. However, none of the methods proposed so far are considered to be effective for the motor scrap of interest and, hence, have not been commercialized.
Under the circumstances, most of the motor scrap are disposed of by sanitary landfill without being processed in any way. However, as huge amounts of motor scrap are discharged every year, it is becoming increasingly difficult to find suitable landfill sites, thus presenting a serious social problem.
Entirely the same problem has occurred with shredded wastes of automobiles and home electric appliances such as washing machines and refrigerators. A recent method for processing such industrial solid wastes or scrap home electric appliances consists of shredding, followed by combined magnetic separation and handsorting to recover iron scrap, as well as disposal by sanitary landfill of shredder dust of low density which is chiefly composed of plastics, paper, fabrics, etc. In magnetic separation, some of the mixtures that are based on ferrous and/or nonferrous metals are contained in the nonmagnetics and yet they cannot be recovered by handsorting. Examples of such difficult-to-recover solid wastes are shredded enamelled copper wires and metallic machines coated with insultation paint. In the absence of effective processing techniques, these solid wastes are just left as they are or dumped at suitable sites.
The magnetics that have been recovered from the shredded solid wastes by magnetic separation include not only those which can be separated from iron scrap by subsequent handsorting but also dust, as well as aggregates of ferrous and nonferrous metals that adhere to each other, and mixtures thereof. The undersize of nonmagnetics contains copper-rich ferrous/nonferrous mixed scrap. Further, the heavy fractions recovered by air classification contain copper, aluminum and iron and are important valuable metal resources with comparatively small particle sizes. Some of these are recoverable as electric wire scrap but their quantity is extremely small. Thus, except for iron scrap, no appropriate techniques for processing industrial solid wastes and scrap home electric appliances have been known and there is a strong need to establish an effective system for their recycling.
Among the industrial solid wastes and scrap home electric appliances described hereinabove, motor scrap of common grades is chiefly composed of copper and iron and, among the shredded solid wastes also mentioned above, the inventors of the present invention are particularly interested in mixtures of iron and copper or other nonferrous metals. Clearly the development of an effective method of processing those industrial solid wastes and scrap home electric appliances is urgent for countries which are heavily dependent on overseas countries for the supply of raw mineral ores of ferrous and nonferrous metals, such as Japan. However, in view of the low price of the wastes to be processed, it is necessary to develop a process that is sufficiently inexpensive and convenient to warrant its commercial application.
Under the circumstances, the present inventors conducted intensive studies in order to develop and establish an effective and appropriate method for processing metallic industrial solid wastes and scrap home electric appliances that is simple to operate, that requires lower materials and energy costs, and that yet has a large disposal capacity. The present invention has been accomplished as a result of these studies.